Water electrolysis system and method of operating water electrolysis system

ABSTRACT

A water electrolysis system includes a water electrolysis apparatus, a gas-liquid separating apparatus, a hydrogen supply line, a water level detector, and a current regulator. The water electrolysis apparatus is to electrolyze water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen. The gas-liquid separating apparatus is disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen. The hydrogen supply line is to supply the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus. The water level detector is configured to detect a water level in the gas-liquid separating apparatus. The current regulator is configured to regulate a current to be applied to the water electrolysis apparatus based on the water level detected by the water level detector in the gas-liquid separating apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Applications No. 2011-083944 filed Apr. 5, 2011, entitled “Water Electrolysis System and Method of Operating Same.” The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a water electrolysis system, and a method of operating the water electrolysis system.

2. Discussion of the Background

Generally, hydrogen is used as a fuel gas to be used in the power generation reaction of a fuel cell. This hydrogen gas is generated by a water electrolysis apparatus. The water electrolysis apparatus employs a solid polymer electrolyte membrane (ion exchange membrane) for decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, forming an electrolyte membrane/electrode assembly. Current collectors are disposed on the respective sides of the electrolyte membrane/electrode assembly, making up a unit cell.

A plurality of such units are stacked to form a cell unit. A voltage is applied across the cell unit while water is supplied to the current collectors on the anode side. On the anodes of the electrolyte membrane/electrode assembly, water is decomposed to produce hydrogen ions (protons). The hydrogen ions move through the solid polymer electrolyte membranes to the cathodes to be combined with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the cell unit.

The water electrolysis apparatus produces hydrogen containing moisture which needs to be removed therefrom to provide a dry state, i.e., to obtain hydrogen with a moisture content of, for example, 5 ppm or less (hereinafter also referred to as “dry hydrogen”).

A high-pressure hydrogen producing apparatus which obtains high pressure (e.g., 1 MPa or higher) hydrogen having a higher pressure than oxygen on the cathode side has a problem that the size of a gas-liquid separating apparatus for removing moisture from high-pressure hydrogen is enlarged.

As a solution to the problem, there is a gas-liquid separating apparatus disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2006-347779. As shown in FIG. 5, the gas-liquid separating apparatus includes a pressure resistant container 2 connected with a hydrogen pipe 1, a water level sensor 3 which detects a water level in the pressure resistant container 2, a hydrogen extraction pipe 4 a as hydrogen extraction means 4 connected to the ceiling portion of the pressure resistant container 2, and a water discharge pipe 5 a as water discharge means 5 connected to the bottom of the pressure resistant container 2.

The hydrogen extraction pipe 4 a is provided with a first back pressure valve 6 and a solenoid valve 7 at the downstream of the first back pressure valve 6. The water discharge pipe 5 a is provided with a second back pressure valve 8.

The first back pressure valve 6 is set to open at a pressure of, for example, 35 MPa, and the second back pressure valve 8 is set to open at a pressure higher than the pressure of the first back pressure valve 6, for example, 36 MPa. The solenoid valve 7 is actuated in response to a detection signal from the water level sensor 3. The solenoid valve 7 opens when the water level detected by the water level sensor 3 becomes a predetermined low level, and closes when the water level becomes a predetermined high level.

When the solenoid valve 7 is closed, extraction of high-pressure hydrogen from the hydrogen extraction pipe 4 a is forcibly stopped, so that the pressure inside the pressure resistant container 2 becomes higher than 35 MPa or the set pressure of the first back pressure valve 6. As a result, the second back pressure valve 8 opens every time the pressure in the pressure resistant container 2 becomes higher than 36 MPa or the set pressure thereof, causing liquid water to be intermittently discharged from the water discharge pipe 5 a through the second back pressure valve 8.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a water electrolysis system includes a water electrolysis apparatus, a gas-liquid separating apparatus, a hydrogen supply line, a water level detector, and a current regulator. The water electrolysis apparatus is to electrolyze water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen. The gas-liquid separating apparatus is disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen. The hydrogen supply line is to supply the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus. The water level detector is configured to detect a water level in the gas-liquid separating apparatus. The current regulator is configured to regulate a current to be applied to the water electrolysis apparatus based on the water level detected by the water level detector in the gas-liquid separating apparatus.

According to another aspect of the present invention, a method is for operating a water electrolysis system including a water electrolysis apparatus to electrolyze water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen, a gas-liquid separating apparatus disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen, a hydrogen supply line to supply the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus, a water level detector configured to detect a water level in the gas-liquid separating apparatus, and a current regulator configured to regulate a current to be applied to the water electrolysis apparatus. The method includes detecting the water level in the gas-liquid separating apparatus using the water level detector, and limiting the current to be applied to the water electrolysis apparatus to or lower than a specified current value when it is determined that the water level detected by the water level detector in the gas-liquid separating apparatus is equal to or higher than a specified water level.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 is a diagram illustrating the schematic configuration of a water electrolysis system according to the embodiment.

FIG. 2 is a diagram showing the relation among the amount of permeating water, the temperature and an electrolysis current.

FIG. 3 is a timing chart for a method of operating the water electrolysis system according to a first embodiment.

FIG. 4 is a timing chart for a method of operating the water electrolysis system according to a second embodiment.

FIG. 5 is an explanatory diagram of a gas-liquid separating apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2006-347779.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.

As shown in FIG. 1, a water electrolysis system 10 according to a first embodiment includes a differential-pressure type water electrolysis apparatus (high-pressure hydrogen producing apparatus) 12 that electrolyzes water (pure water) to produce oxygen and high-pressure hydrogen (hydrogen with a pressure higher than the oxygen pressure or the normal pressure of, for example, 1 MPa to 70 MPa), a water reservoir device 14 that separates the oxygen and excess water discharged from the water electrolysis apparatus 12 from each other and stores the water, a water circulating apparatus 16 that circulates the water stored in the water reservoir device 14 to the water electrolysis apparatus 12, a water supply device 18 that supplies pure water produced from tap water to the water reservoir device 14, a gas-liquid separating apparatus 22 that removes moisture contained in the high-pressure hydrogen supplied to a hydrogen supply line 20 from the water electrolysis apparatus 12, a high-pressure hydrogen discharge line 24 for discharging the water-separated high-pressure hydrogen from the water electrolysis apparatus 12, and a controller (control apparatus) 28.

The water electrolysis apparatus 12 includes a cell unit having a plurality of stacked unit cells 30. The water electrolysis apparatus 12 also includes a terminal plate 32 a, an insulating plate 34 a, and an end plate 36 a which are mounted on an end of the unit cell 30 in the stacking direction outward in the order named, and a terminal plate 32 b, an insulating plate 34 b. The water electrolysis apparatus 12 further includes a terminal plate 32 b, an insulating plate 34 b and an end plate 36 b which are likewise mounted on the other end of the unit cell 30 in the stacking direction outward in the order named. The components disposed between the end plates 36 a, 36 b are integrally fastened together.

Terminals 38 a, 38 b project outward from respective side portions of the terminal plates 32 a, 32 b. The terminals 38 a, 38 b are electrically connected to a DC variable power supply 40 by electric wires 39 a, 39 b, respectively. The DC variable power supply 40 and the controller 28 constitute a current regulator that regulates an electrolysis current to be applied to the water electrolysis apparatus 12.

Each of the unit cells 30 includes a disk-shaped electrolyte membrane/electrode assembly 42, and an anode separator 44 and a cathode separator 46 which sandwich the electrolyte membrane/electrode assembly 42 therebetween. Each of the anode separator 44 and the cathode separator 46 has a disk shape.

The electrolyte membrane/electrode assembly 42 has a solid polymer electrolyte membrane 48 having a thin membrane of, for example, perfluorosulfonic acid which is impregnated with water, and an anode current collector 50 and a cathode current collector 52 which are disposed respectively on the opposite surfaces of the solid polymer electrolyte membrane 48.

An anode electrode catalyst layer 50 a and a cathode electrode catalyst layer 52 a are formed on the opposite surfaces of the solid polymer electrolyte membrane 48, respectively. The anode electrode catalyst layer 50 a is made of a Ru (ruthenium)-based catalyst, for example, and the cathode electrode catalyst layer 52 a of a platinum catalyst, for example.

The unit cells 30 have water supply passages 56, discharge passages 58 and hydrogen feed passages 60 formed in the outer peripheral edges thereof. The water supply passages 56 communicate with one another in the stacking direction to supply water (pure water). The discharge passages 58 communicate with one another in the stacking direction to discharge oxygen generated by a reaction and unreacted water (mixed fluid). The hydrogen feed passages 60 communicate with one another in the stacking direction to discharge hydrogen generated by a reaction.

A first flow passage 64 which communicates with the water supply passage 56 and the discharge passage 58 is provided in a surface of each of the anode separators 44 which faces the electrolyte membrane/electrode assembly 42. The first flow passage 64 is provided in a range corresponding to the surface area of the anode current collector 50. The first flow passage 64 includes a plurality of flow passage grooves, a plurality of embossed ridges, or the like. Oxygen generated by a reaction and unreacted water flow through the first flow passage 64.

A second flow passage 68 which communicates with the hydrogen feed passage 60 is formed in a surface of each of the cathode separators 46 which faces the electrolyte membrane/electrode assembly 42. The second flow passage 68 is provided in a range corresponding to the surface area of the cathode current collector 52. The second flow passage 68 includes a plurality of flow passage grooves, a plurality of embossed ridges, or the like. High-pressure hydrogen generated by a reaction flows through the second flow passage 68.

The water circulating apparatus 16 includes a circulation pipe 72 communicating with the water supply passages 56 of the water electrolysis apparatus 12. The circulation pipe 72 is connected to the bottom of a tank 78 where a circulation pump 74 and an ion exchanger 76 are disposed to form the water reservoir device 14.

The tank 78 has a top portion connected with one end of a return pipe 80 whose opposite end is connected to the discharge passages 58 of the water electrolysis apparatus 12. The one end of the return pipe 80 is set to a position in which the end is normally open in water stored in the tank 78.

The tank 78 is also connected with a pure water supply pipe 84 which is connected to the water supply device 18 and an oxygen discharge pipe 86 for discharging oxygen separated from pure water in the tank 78.

The hydrogen feed passages 60 of the water electrolysis apparatus 12 are connected with one end of the hydrogen supply line 20 whose opposite end is connected to the bottom of the gas-liquid separating apparatus 22. The gas-liquid separating apparatus 22 is disposed upstream of the water electrolysis apparatus 12 in the gravitational direction. More specifically, the lower end position of the gas-liquid separating apparatus 22 (the bottom of a tank 88) is located above the upper end position of the water electrolysis apparatus 12 (the top surface of the end plate 36 a).

The gas-liquid separating apparatus 22 has the tank 88 for storing water. The tank 88 is provided with a water level sensor (water level detector) 90 that detects whether a water level WS in the tank 88 lies between a lower water level threshold L and an upper water level threshold (specified water level) H. A detection signal from the water level sensor 90 is input to the controller 28 which is also supplied with the operational temperature of the water electrolysis apparatus 12 which is detected by a temperature sensor 92 mounted to the water electrolysis apparatus 12.

High-pressure hydrogen having moisture removed by the gas-liquid separating apparatus 22 is supplied as dry hydrogen to the high-pressure hydrogen discharge line 24. The high-pressure hydrogen discharge line 24 is provided with a back pressure valve 94 whose pressure is set to a set pressure value (e.g., 35 MPa).

The operation of the water electrolysis system 10 with the foregoing configuration is described below.

First, when the water electrolysis system 10 is activated, pure water produced from tap water by the water supply device 18 is supplied to the tank 78 constituting the water reservoir device 14.

In the water reservoir device 14, with the circulation pump 74 actuated, the water in the tank 78 is supplied through the circulation pipe 72 to the water supply passages 56 of the water electrolysis apparatus 12. A voltage is applied between the terminals 38 a, 38 b of the terminal plates 32 a, 32 b by the DC variable power supply 40 electrically connected to the terminals 38 a, 38 b.

In each unit cell 30, therefore, the water is supplied from the water supply passage 56 into the first flow passage 64 of the anode separator 44, and moves along the anode current collector 50.

Therefore, the water is electrolyzed by the anode electrode catalyst layer 50 a, generating hydrogen ions, electrons, and oxygen. The hydrogen ions generated by the anodic reaction move through the solid polymer electrolyte membrane 48 to the cathode electrode catalyst layer 52 a where they combine with the electrons to produce hydrogen.

The produced hydrogen flows along the second flow passage 68 that is formed between the cathode separator 46 and the cathode current collector 52. The hydrogen is kept at a pressure higher than the pressure in the water supply passage 56, and flows through the hydrogen feed passage 60 to be extractable from the water electrolysis apparatus 12.

The oxygen generated by the anodic reaction and the unreacted water flow in the first flow passage 64, and the mixed fluid of the oxygen and the unreacted water flows through the discharge passage 58 to be discharged into the return pipe 80 of the water circulating apparatus 16. The unreacted water and the oxygen are introduced into the tank 78 where they are separated from each other. The water is supplied from the circulation pipe 72 through the ion exchanger 76 into the water supply passages 56 by the circulation pump 74. The oxygen separated from the water is discharged through the oxygen discharge pipe 86.

The hydrogen generated in the water electrolysis apparatus 12 is fed to the gas-liquid separating apparatus 22 through the hydrogen supply line 20. In the gas-liquid separating apparatus 22, water vapor (moisture) contained in the hydrogen, which moves from the anode side to the cathode side, is separated from the hydrogen and stored in the tank 88, while the hydrogen is supplied to the high-pressure hydrogen discharge line 24.

The amount of water permeation from the anode side to the cathode side at the time of electrolyzation in the unit cell 30 in the water electrolysis apparatus 12 is acquired from the electrolysis current and the operational temperature in the electrolyzation. That is, as shown in FIG. 2, there are characteristics such that the higher the electrolysis current c (A) becomes, the greater the amount of water permeation b (cc/min) becomes, and the higher the operational temperature a (° C.) becomes, the smaller the amount of water permeation b (cc/min) becomes.

In view of the characteristics, as shown in FIG. 3, the electrolysis current is determined based on the water level WS in the tank 88 in operation and the operational temperature of the water electrolysis apparatus 12. Specifically, the water level WS in the tank 88 is detected by the water level sensor 90 in the gas-liquid separating apparatus 22. The controller 28 carries out an operation on the electrolysis current with the specified current value when having determined that the water level WS lies between the lower water level threshold L and the upper water level threshold H.

When having determined that the water level WS will exceed the upper water level threshold H, the controller 28 limits the electrolysis current to the specified current value or less. As a result, in the water electrolysis apparatus 12, the amount of moisture moving from the anode side to the cathode side decreases, so that the amount of moisture permeation becomes less than the amount of moisture returned to the anode side from the cathode side, and water is substantially returned to the anode side from the cathode side. Accordingly, water is returned to the water electrolysis apparatus 12 from the tank 88 of the gas-liquid separating apparatus 22 disposed upstream of the water electrolysis apparatus 12 in the gravitational direction. As a result, the water level WS in the tank 88 falls, and when the water level WS reaches the lower water level threshold L, the electrolysis current is increased to the specified current value. This suppresses the return of water from the cathode side to the anode side, promoting the supply of water to the tank 88.

When the operational temperature of the water electrolysis apparatus 12 becomes higher, the amount of water permeation from the anode side to the cathode side decreases. Therefore, the operational time on the electrolysis current with the specified current value is set to become longer when the operational temperature of the water electrolysis apparatus 12 becomes higher.

According to the first embodiment, as apparent from the above, the water level WS in the gas-liquid separating apparatus 22 is detected, and the electrolysis current to be applied to the water electrolysis apparatus 12 is regulated based on the detected water level in the gas-liquid separating apparatus 22.

Because the water level WS in the gas-liquid separating apparatus 22 can be controlled merely by regulating the electrolysis current, it is unnecessary to provide a special water discharge structure in the gas-liquid separating apparatus 22. This brings about an effect of eliminating the need for a water discharge structure in the gas-liquid separating apparatus 22 and achieving efficient water electrolysis with a simple configuration and simple process.

FIG. 4 is a timing chart for a method of operating the water electrolysis system according to a second embodiment.

The electrolysis current is intermittently controlled in the first embodiment as shown in FIG. 3, whereas fine control of electrolysis current is continuously carried out to keep the water level WS to a constant water level control value in the second embodiment.

Therefore, the second embodiment brings about effects similar to those of the first embodiment, such as a need to control the electrolysis current, and having a simple configuration and a simple process to eliminate the need for a dedicated water discharge structure.

A water electrolysis system according to the embodiment includes a water electrolysis apparatus that electrolyzes water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen, a gas-liquid separating apparatus disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen, a hydrogen supply line for supplying the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus, a water level detector that detects a water level in the gas-liquid separating apparatus, and a current regulator that regulates a current to be applied to the water electrolysis apparatus based on the detected water level in the gas-liquid separating apparatus.

A method of operating a water electrolysis system according to the embodiment includes a water electrolysis apparatus that electrolyzes water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen, a gas-liquid separating apparatus disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen, a hydrogen supply line for supplying the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus, a water level detector that detects a water level in the gas-liquid separating apparatus, and a current regulator that regulates a current to be applied to the water electrolysis apparatus.

The method includes the steps of detecting the water level in the gas-liquid separating apparatus using the water level detector, and limiting the current to be applied to the water electrolysis apparatus to or lower than a specified current value when it is determined that the detected water level in the gas-liquid separating apparatus is equal to or higher than a specified water level.

Preferably, the temperature of the water electrolysis apparatus is detected, and the specified current value is set based on the detected temperature of the water electrolysis apparatus.

According to the embodiment, the water level in the gas-liquid separating apparatus is detected, and the current to be applied to the water electrolysis apparatus is regulated based on the detected water level in the gas-liquid separating apparatus. When the water electrolysis apparatus is operated on a high current, the amount of moisture moving from the anode side to the cathode side becomes larger than the amount of moisture that is returned to the anode side from the cathode side. When the water electrolysis apparatus is operated on a low current equal to or lower than a given current, on the other hand, the amount of moisture that is returned from the cathode side to the anode side or a low-pressure side becomes larger than the amount of moisture moving from the anode side to the cathode side.

When it is determined that the detected water level in the gas-liquid separating apparatus is equal to or higher than a specified water level, therefore, the current to be applied to the water electrolysis apparatus is limited to a specified current value or lower to make the amount of returning moisture larger than the amount of permeating moisture, so that water on the cathode side is substantially returned to the anode side.

This system can control the water level in the gas-liquid separating apparatus, thus eliminating the need for providing a special water discharge structure in the gas-liquid separating apparatus. Therefore, an efficient water electrolysis process can be carried out with a simple configuration and a simple process.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

1. A water electrolysis system comprising: a water electrolysis apparatus to electrolyze water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen; a gas-liquid separating apparatus disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen; a hydrogen supply line to supply the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus; a water level detector configured to detect a water level in the gas-liquid separating apparatus; and a current regulator configured to regulate a current to be applied to the water electrolysis apparatus based on the water level detected by the water level detector in the gas- liquid separating apparatus.
 2. The water electrolysis system according to claim 1, wherein the current regulator regulates the current to be applied to the water electrolysis apparatus to a current value lower than a predetermined current value when the water level detected by the water level detector exceeds an upper water level.
 3. The water electrolysis system according to claim 2, wherein the current regulator increases the current to be applied to the water electrolysis apparatus to the predetermined current value when the water level detected by the water level detector reaches a lower water level lower than the upper water level.
 4. The water electrolysis system according to claim 3, further comprising: a temperature detector configured to detect a temperature of the water electrolysis apparatus, wherein the current regulator is configured to set the predetermined current value based on the temperature detected by the temperature detector.
 5. The water electrolysis system according to claim 2, further comprising: a temperature detector configured to detect a temperature of the water electrolysis apparatus, wherein the current regulator is configured to set the predetermined current value based on the temperature detected by the temperature detector.
 6. A method of operating a water electrolysis system including a water electrolysis apparatus to electrolyze water to generate oxygen and high-pressure hydrogen having a higher pressure than a pressure of the oxygen, a gas-liquid separating apparatus disposed upstream of the water electrolysis apparatus in a gravitational direction to separate moisture contained in the high-pressure hydrogen, a hydrogen supply line to supply the high-pressure hydrogen discharged from the water electrolysis apparatus to the gas-liquid separating apparatus, a water level detector configured to detect a water level in the gas-liquid separating apparatus, and a current regulator configured to regulate a current to be applied to the water electrolysis apparatus, the method comprising: detecting the water level in the gas-liquid separating apparatus using the water level detector; and limiting the current to be applied to the water electrolysis apparatus to or lower than a specified current value when it is determined that the water level detected by the water level detector in the gas-liquid separating apparatus is equal to or higher than a specified water level.
 7. The method according to claim 6, further comprising: detecting a temperature of the water electrolysis apparatus using a temperature detector; and setting the specified current value based on the temperature detected by the temperature detector. 